Add STM32_CHIP to the cache if it's specified.

Fixes re-cmakes later on.

cmake/gcc_stm32.cmake

@@ -2,6 +2,9 @@ GET_FILENAME_COMPONENT(STM32_CMAKE_DIR ${CMAKE_CURRENT_LIST_FILE} DIRECTORY)
 SET(CMAKE_MODULE_PATH ${STM32_CMAKE_DIR} ${CMAKE_MODULE_PATH})
 
 SET(STM32_SUPPORTED_FAMILIES L0 F0 F1 F2 F3 F4 F7 CACHE INTERNAL "stm32 supported families")
+IF(STM32_CHIP)
+	 SET(STM32_CHIP "${STM32_CHIP}" CACHE STRING "STM32 chip to build for")
+ENDIF()
 
 IF(NOT TOOLCHAIN_PREFIX)
      SET(TOOLCHAIN_PREFIX "/usr")